Thermal operator for ignition proving system

ABSTRACT

An improved operator is disclosed which greatly amplifies the relatively small expansion and contraction movements of a heat expansible wire connected in series with an electric igniter having a positive coefficient of resistance. As the igniter changes temperature its electrical resistance also changes causing a difference in current flow through the wire and accordingly an expansion or contraction thereof. Lever means attached to the wire amplify this movement and utilize it to selectively actuate switches controlling energization of both the electric igniter and an electrically actuated fuel valve providing fuel to a burner.

United States Patent 1 Clifford [451 Sept. 18,1973

THERMAL OPERATOR FOR IGNITION PROVING SYSTEM [75] Inventor: JosephClifford, Corona, Calif.

[73] Assignee: Robertshaw Controls Company,

Richmond, Va.

[22] Filed: Nov. 12, 1971 [21] Appl. No.: 198,142

[52] 0.8. Cl. 431/66, 431/73 [51] Int. Cl. F23n 5/00 [58] Field ofSearch 431/66, 67, 70, 73

[56'] I References Cited UNITED STATES PATENTS 3,619,096 11/1977]Krueger et al. 431/66 3,660,005

5/1972 "Wilson 431/66 Primary Examiner-Carroll B. Dority/Jr.Attorney-Anthony A. OBrien [57] ABSTRACT An improved operator isdisclosed which greatly amplifies the relatively small expansion andcontraction movements of a heat expansible wire connected in series withan electric igniter having a positive coefficient of resistance. As theigniter changes temperature its electrical resistance also changescausing a difference in current flow through the wire and accordingly anexpansion or contraction thereof. Lever means attached to, the wireamplify this movement and utilize it to selectively actuate switchescontrolling energization of both'the electric igniter and anelectrically actuated fuel valve providing fuel to a burner.

5 Claims, 3 Drawing Figures BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention pertains to an electric ignitionmechanism and more particularly to a thermally responsive actuatorcapable of sensing minor movements of a hot wire and amplify thesemovements sufficiently to provide a useful function.

2. Description of the Prior Art A recent trend in the field of ignitingdevices for fuel burners is to utilize electric igniters in place of thewellknown standing pilot flames. Bumer'operation, it has been learned,can be monitored by this type of igniter if the igniter element isconstructed of a material having a positive coefficient of resistance,namely a resistance which varies with temperature. The resistance ofsuch an igniter varies as a function of the temperature and by sensingthe changing resistance, the operation of the igniter and burner may becontrolled.

lnherently dangerous ,conditionscan arise in fuel burners resulting fromimproper operation of ignition systems. It is necessary that the systemsbe capable of being shut down and fuel flow stopped upon the failure ofany system component or interruption of electric power to the system.More specifically, it is important that the valve supplying fuel to theburner be open onlywhen igniting temperatures exist to prevent abuild-up or raw fuel. It is also desirable to have the system recyclewhen igniting temperatures are not timely provided, once electricity isrestored after an interruption or when the valve is closed due to otherfailures in the system in order to have another attempt made to initiateignition when the system is in proper operation.

The resistance of electric igniters has been used in previously knownelectric ignition systems, but such known systems have lackedsophistication and have not provided the required comprehensive failsafeor recycling characteristics. Furthermore, the known ignition systemshave not taken full advantage of the temperature variable resistancecharacteristics'ofthe electric igniter element.

SUMMARY OF THE INVENTION Accordingly, it is an object of the presentinvention to construct a novel electric ignition mechanism utilizing theelectrical characteristics of an electric igniter which vary withtemperature to provide'a controlling function.

Another object of the present invention is to utilize an electricigniter having a positivetemperature coefficient of resistance as asensor and control element as well as for ignition purposes.

A further object of the present invention is to provide an electricignition mechanism having electrical sensing means connected to anigniter to sense changing electrical characteristics and controloperation of a fuel valve in response to these changes. A

Yet another object of the present invention is to provide a thermallyexpansive wire connected in series with an igniter, to be heated by thepassage of current therethrough, and a lever attached to the wire in amanner which will greatly amplify the expansion and contractionmovements of the wire to perform useful work in a control function.

The present invention is intended for use with an ignition provingsystem for a fuel burner comprising a burner, an electric igniter meanswith a positive temperature coefficient of resistance in ignitingproximity to the burner, and an electrically actuated valve connectingthe burner to a source of fuel. The invention is characterized by acurrent sensing thermally expansive BRIEF DESCRIPTION OF THE DRAWINGSFIG. l is a schematic diagram of an electric ignitor mechanism embodyingthe present invention and showing the mechanism in a ready condition;

FIG. 2 is a schematic diagram similar to FIG. 1 showing the mechanismafter successful ignition; and

FIGS is a schematic diagram of the mechanism shown in FIG. l but withthe mechanism shown in a fail-safe state. i I

I DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment ofthe present thermal operator for an ignition proving system isillustrated in FIGS. 1 through 3 for use with a fuel burner 10. Anelectrically operated fuel valve 12 connects burner 10 with source offuel, not shown, to control the flow of fuel to burner 10. The valve 12may be controlled by any suitable electric operator, such as a solenoid.An electric ignitor 14 is disposed in igniting proximity to the ports ofburner 10. The ignitor14 is constructed of material havinga positivetemperature coefficient of resistance, such as molybdenum disilicide. Ahousing 16, shown in broken lines, has a pair of input terminals 18 and20 connected with secondary winding 22 of a transformer 24 having aprimary winding 26 connected to a suitable source of electric power, notshown, through a thermostatic switch 28. Terminal 18 is connected to oneend of a stationary contact 30 which is secured to housing 16 andcarries a fixed contact 32 on the other end. A movable contact 34cooperates with fixed contact 32 to form a switch generally indicated at36.

Movable contact 34 is carried by a movable contact arm 38 which has oneend pivotably mounted on a pin' 54. Depending leg 56 of T-shaped lever52 engages a mid point of an electrically conductive hot wire 58 mountedbetween insulated portions 60 and 62 of housing 16 with one end of wire58 electrically connected with terminal 20. Hot wire 58 has a thermalexpansion characteristic such that it expands as it heats and contractsas it cools. At normal room temperature the hotwire 58 has a relativelycontracted state, as shown in FIG. 1.

A helical bias spring 64 is mounted in compression between housing 16and the free end of lever 52. An upstanding finger 66 is secured to theupper side of lever 52 and has a rounded free end 68 adapted to abut theunderside of movable contact arm 38. A disc 70 is fixed adjacent thefree end of finger 66 and is adapted to abut the underside of a secondmovable contact arm 72 which is pivotably mounted in housing 16 on a pin74. A bias spring 76 is mounted in compression between the underside offirst movable contact arm 38 and the upper side of second movablecontact arm 72. The second movable contact arm 72 carries on itsunderside a movable contact 78 which cooperates with a fixed contact 80to form a second switch generally indicated at 82. Fixed contact 80 iscarried on one end of second stationary contact 84 which is fixed inhousing 16.

Housing 16 has three output terminals 86,88 and 90. Terminal 86 iselectrically connected with movable contact arms 38 and 72 within thehousing 16 and is connected through a lead 92 to terminal 94 of ignitor14. The other terminal 96 of ignitor 14 is connected to a terminal 88through lead 98, and terminal 88 is connected with hot wire 58 withinthe housing 16 through a lead 100. Terminal 90 is connected to secondstationary contact 84 within housing 16 and is connected through a lead102 to a terminal 104 of fuel valve 12. The other terminal 106 of fuelvalve 12 is connected through lead 108 to input terminal 20.

Holding coil 1 has two legs which are connected in parallel with ignitor14 through leads 112 and 114. The center tap 116 for coil 110 is coupledto lead 112 through an input terminal 118 of housing 16. Lead 114 isconnected through an input terminal 120 of housing 16 and oppositelypoled full-wave rectifying diodes 122 and 124 to the two legs of coil110. An armature 126 is controlled by coil 110 and is secured to theupper side of movable contact arm 38.

In operation, when thermostatic switch 28 is open, the ignition provingsystem will be in a ready state, as illustrated in FIG. 1, with no powersupplied to the system, fuel valve 12 closed, and ignitor 14deenergized. I-Iot wire 58 will be cold since no current will be flowingtherethrough and will therefore be contracted to hold lever 52 upagainst the force of helical bias spring 64 and finger 66 will holdcontact arm 38 up against the force of helical bias spring 42 to closecontacts 32 and 34 of switch 36. Contacts 78 and 80 of switch 82 will beopen at this time due to contact arm 72 engaging with the disc 70 onfinger 66.

Once the thermostatic switch 28 is closed, signalling a call for burneroperation, current will flow with terminal 18 through closed switch 36,lead 92, ignitor 14, leads 98 and 100, and hot wire 58 to terminal toenergize ignitor l4 and begin heating hot wire 58. Since ignitor 14 hasa positive temperature coefficient of resistance, the initial voltagedrop across it is small and not sufficient to pull in coil 110 so thatmost of the initial voltage drop will be across hot wire 58. Coil 110must be very sensitive to voltage across ignitor 14 and therefore isarranged so that it does not have to pull in armature 126 to closeswitch 36 but instead merely holds armature 126, since switch 36 isclosed at the start of operation.

The helical bias spring '64 urges lever 52 to rotate in clockwisedirection about pivot 54 thereby holding the depending leg 56 firmlyagainst the hot wire 58. When current is passed through terminal 88,lead and hot wire 58 to terminal 20, the hot wire 58 expands and allowslever 52 to rotate under the influence of spring 64 in a clockwisedirection about pivot 54.

The rotational movement of lever 52 moves finger 66 downward allowingcontact arm 72 to be pivoted clockwise about pivot 74 by spring 76 andcontact arm 38 is free to pivot clockwise about pivot 40, as shown inFIG. 2 which illustrates the state of the ignition proving system aftersuccessful ignition. If ignitor 14 has reached ignition temperature bythis time, end 68 of finger 66 moves away from movable contact arm 38,the resistance of ignitor 14 will be high due to the ignitors positivetemperature coefficient of resistance and, accordingly, the voltage dropthereacross will be sufficient to energize holding coil and maintainswitch 36 closed. Since coil 110 is connected in parallel with ignitor14, it experiences the same voltage drop and thus coil 110 will besufficiently energized to hold armature 126 only when ignitor 14 is atignition temperature. Diodes 122 and 124 provide full wave rectifiedcurrent to holding coil 110 to eliminate noise and chatter when the coilis near its dropout point.

Switch 82 is closed by the clockwise movement of contact arm 72, andvalve 12 is energized by current flowing from terminal 18 through closedswitches 36 and 82 and leads 102 and 108 to terminal 20. Thus, whenignitor 14 reaches ignition temperature within the predetermined timebetween closure of thermostatic switch 28 and expansion of hot wire 58,valve 12 will be open to supply fuel to burner 12.

Iffor any reason ignitor 14 fails to reach ignition temperature by thetime the current through hot wire 58 heats the wire sufficiently for itsexpansion to permit spring 64 to move lever 52 clockwise, switch 36 willbe opened since contact arm 38 will follow finger 66 due to the forcefrom bias spring 42, see FIG. 3. Once switch 36 is opened, all electricpower from second winding 22 is interrupted and, accordingly, ignitor 14is de-energized and valve 12 is closed. Thus, it can be seen that valve12 depends on the operation of both switches 36 and 82, which are inseries, and that valve 12 will not open unless ignitor 14 is at ignitiontemperature. Springs 42 and 76 are selected so that contacts 32 and 34of switch 36 will break prior to the making of contacts 78 and 80 ofswitch 82, thereby assuring that upon failure of ignitor 14 to reachignition temperature within the predetermined time, valve 12 will not beopened.

Opening of switch 36 also prevents current from flowing through hot wire58 thereby permitting hot wire 58 to cool and contract moving lever 52counterclockwise about pivot 54 and finger 66 to close contacts 32 and34 of switch 36. Thus, the system is returned to the ready state shownin FIG. 1 and will recycle until ignitor l4 properly reaches ignitiontemperature within the predetermined time, assuming thermostatic switch28 remains closed.

If during normal operation of the ignition proving system any type ofelectrical interruption occurs, holding coil 110 will be de-energized todrop out armature 126 and permit the force from bias spring 42 to openswitch 36 thereby deenergizing ignitor 14, closing valve 12 and stoppingthe flow of current through hot wire 58. The system is thus placed infail-safe state as shown in FIG. 3.

Since no current passes through hot wire 58 in this I state, the wirewill cool and contract causing lever 52 to move counterclockwise toplace the system in the ready state shown in FIG. 1 and thus permitrecycling, again assuming thermostatic switch 28 remains closed whenpower is restored. The burner will be reignited in the same mannerpreviously described.

If ignitor I4 is ruptured once the system is in the operating stateshown in FIG. 2, the voltage across holding coil 1 will increasesubstantially tohold armature 126 and prevent switch 36 from opening.However, since hot wire 58 is in series with ignitor lid, the currentflowing through hot wire 5% will be interrupted to cool the wire. Aspreviously mentioned, hot wire 58 will contract as it cools to movelever 52 counterclockwise and cause finger 66 to lift contact arm 72 andopen switch 82 thereby closing valve 12 and stop flow of fuel to theburner. The system will remain in this condition until ignitor M isreplaced.

If ignitor 14 should become short-circuited, the voltage there acrosswill decrease to zero'and coil 110 will allow armature 126 to drop outtoopen switch 36 and close valve 12. If hot wire 58 breaks, the currentpath through ignitor 14 will be opened to allow holding coil 110 todropout armature 126 and permit contact arm 38 to follow lever 52 underthe force of spring 42 to open switch 36. If a short-circuit occursacross hot wire 58, the wire will cool and contract thereby maintainingswitch 82 open due to the lifting of contact arm 72 by disc 70. i

In the event of a failure due to ignitor 14 being cooled, such as by thefuel stream, the voltage across the ignitor l4 and holding coil 110 willbe reduced due to the drop in resistanceof ignitor 14 as it is cooled.It has been found that the ignitor cooling can occur to theextent thatignition is prevented while the decreased voltage drop across ignitor I4is insufficient to dropout holding coil 110. In order to make the systemof the present invention fail safe for such ignitor cooling, spring 48is mounted in tension between lever 52 and movable contact arm 38. Thedecrease in the resistance of ignitor14 upon cooling causes the currentflowing through hot wire 58 to increase and this increase causes furtherheating and expansion of the hot wire thus permitting lever 52 toincrease its movement in the clockwise direction. This increasedmovement' of lever 52 stretches spring 48 to increase the clockwiseforce on movable contact arm 38 until armature 126 is released byholding coil 110, even though the voltage across holding coil 110 is ata higher than normal value. Thus, it can be seen that the system of thepresent invention is extremely sensitive to external conditions thatmight cool ignitor M since armature l26responds to the voltage acrossignitor M and the current through ignitor 14. The system of the presentinvention can thus assume a fail safe state in response to internal orexternal thereacross to provide fail-safe operation. It may also be seenthat the present invention provides a high mechanical advantagev leversystem which will amplify small hot wire movements that are usuallydifficult to detect and amplify. A relatively low force, large movementis taken as a mechanical output from the side of ahot wire so that theignitior proving system can be operated through a low mechanicaladvantage lever system. Thus, the inventive construction provides anessentially stiff, friction-free mechanical switching arrangement whichsolves the problem of mechanical motion loss due to deflection,hysteresis and mechanical wear.

' The present invention may be embodied in other specific forms withmany variations, modifications and changes in detail. It is thereforeintended that the foregoing description be interpreted as illustrativeand not in a limiting sense.

What is claimed is:

i. in an ignition proving system for a fuel burner comprising inputmeans connected to a source of electricity;

, electric igniter means positioned in igniting proximity to the burner,said electric igniter means having a positive temperature coefficient ofresistance such 7 that current flow therethrough is dependent on thetemperature on the electric igniter;

electrically actuated valve means adapted to control flow of fuel to theburner; and

switch means connected between said input means and both said electricigniter means and said valve means; I

an improved thermal actuator means comprising current sensing meansincluding a heat expansible wire connected in series with said electricigniter to be heated by current passing therethrough; said wire beingattached at each end to a stationary support; and

lever means pivotally connected to a support at one end, said levermeans including an integral leg portion depending from a pointintermediate the ends of said lever means and engaging said wire, andmeans for selectively actuating said switch means in response toexpansion and contractionof said wire.

2. The invention as claimed in claim 1 wherein said wire has a coolcontracted state defining a first position for said lever means and ahot expanded state defining a second position for said lever means,

said switch means including first and second switches, said first switchconnected to energize said igniter means and said first and secondswitches connected in series to energize said valve means, said firstswitch being normally closed and said second switch being normallyopened when said lever means is in said first positionwhereby said valvemeans will not be energized to open until after said igniter meansreaches ignition temperature.

3. The invention as claimed in claim 1 wherein said electric ingitermeans has a high resistance at ignition temperatures, further comprisingvoltage responsive means connected in parallel with said igniter meansand operatively connected to hold said switch means in a predeterminedcondition.

4. The invention as claimed in claim 3 wherein said switch meanscomprises first and second switches, said voltage responsive meansincludes a holding coil and an armature controlled by said coil andconnected to hold said first switch in closed condition.

i t i l i

1. In an ignition proving system for a fuel burner comprising inputmeans connected to a source of electricity; electric igniter meanspositioned in igniting proximity to the burner, said electric ignitermeans having a positive temperature coefficient of resistance such thatcurrent flow therethrough is dependent on the temperature on theelectric igniter; electrically actuated valve means adapted to controlflow of fuel to the burner; and switch means connected between saidinput means and both said electric igniter means and said valve means;an improved thermal actuator means comprising current sensing meansincluding a heat expansible wire connected in series with said electricigniter to be heated by current passing therethrough; said wire beingattached at each end to a stationary support; and lever means pivotallyconnected to a support at one end, said lever means including anintegral leg portion depending from a point intermediate the ends ofsaid lever means and engaging said wire, and means for selectivelyactuating said switch means in response to expansion and contraction ofsaid wire.
 2. The invention as claimed in claim 1 wherein said wire hasa cool contracted state defining a first position for said lever meansand a hot expanded state defining a second position for said levermeans, said switch means including first and second switches, said firstswitch connected to energize said igniter means and said first andsecond switches connected in series to energize said valve means, saidfirst switch being normally closed and said second switch being normallyopened when said lever means is in said first position whereby saidvalve means will not be energized to open until after said igniter meansreaches ignition temperature.
 3. The invention as claimed in claim 1wherein said electric ingiter means has a high resistance at ignitiontemperatures, further comprising voltage responsive means connected inparallel with said igniter means and operatively connected to hold saidswitch means in a predetermined condition.
 4. The invention as claimedin claim 3 wherein said switch means comprises first and secondswitches, said first switch being operatively connected with said levermeans and said voltage responsive means, said second switch beingconnected to be actuated by said lever means only.
 5. The invention asclaimed in claim 3 wherein said voltage responsive means includes aholding coil and an armature controlled by said coil and connected tohold said first switch in closed condition.